1. Fluidized Catalytic Cracking
Crude petroleum is a complex mixture of thousands of different chemical compounds, ranging from methane, which is a dissolved gas, to compounds which are solids at room temperature. These various compounds are composed mostly of hydrogen and carbon, and are accordingly called hydrocarbons. Crude also contains small amounts of sulfur, nitrogen, oxygen, and certain metals. These elements are generally chemically combined with hydrogen and carbon.
When crude petroleum is processed into useful products in an oil refinery, one of the first steps is to separate the crude into various fractions based on boiling point. These fractions, along with their boiling point ranges and respective volume percentages in a typical West Texas crude, are given in Table I.
TABLE I ______________________________________ Crude Petroleum Fractions Boiling Point Fraction Range, .degree.F. Volume Percent ______________________________________ Gas Less than 80 2 Light Naphtha 80-200 11 Heavy Naphtha 200-350 14 Distillate 350-650 17 Gas Oil 650-1000 39 Residual Greater than 1000 17 ______________________________________
The petroleum product for which there is the greatest demand is gasoline, which is itself a complex mixture of hydrocarbons. To burn efficiently in internal combustion engines, gasoline must contain hydrocarbons which have boiling points in the range of about 80.degree. to 400.degree. F. and which also have the right structure to yield a relatively high octane number. The octane number scale is a measure of the ability of a gasoline to resist premature burning or, as it is often called, engine knock. The gasolines sold on the market today have octane numbers in the range of about 85 to 95, on the scale (R+M)/2 where R is the research octane number and M is the motor octane number.
From Table I it can be seen that the oil refiner could use the light naphtha and heavy naphtha fractions to make gasoline since these fractions boil in the 80.degree. to 400.degree. F. range. However, the octane number of these fractions is only about 70, which is too low to be used in today's automobiles. Furthermore, these two fractions make up only about 25 percent of the crude, an amount far below the demand for gasoline relative to other petroleum products.
In response to these needs, the oil refining industry has developed processes to convert non-gasoline-boiling fractions into fractions which boil in the gasoline range and which also have relatively high octane numbers. It has also been discovered that the addition of small amounts of certain antiknock agents, such as tetraethyl lead, substantially increases the octane number of a fraction. For example, the addition of only 3 milliliters of tetraethyl lead solution to a gallon of light naphtha raises the octane number from about 70 to about 85.
The major process for converting non-gasoline-boiling fractions into gasoline-boiling fractions is the fluidized catalytic cracking (FCC) process. In the FCC process, a heavy hydrocarbon fraction, such as a gas oil or a residual, is contacted with hot, finely divided, solid catalyst particles for a period of time sufficient to crack the heavy hydrocarbons into lighter molecular weight products of the kind suitable for gasoline. During the cracking process, a small amount of solid coke is formed on the surface of the catalyst particles. Since the coke reduces the catalytic activity of the particles, it must eventually be removed.
Typically, sufficient contacting time occurs as the hydrocarbons and catalyst flow together up an elongated vertical tube, often called a riser reactor. The riser reactor empties into a large disengaging vessel where the cracked hydrocarbon vapors are separated from the solid catalyst particles. The hydrocarbons are withdrawn to product recovery systems through cyclone separators located within the vessel. The particular configuration of the riser reactor and cyclone separators within the disengaging vessel varies widely in FCC units currently in operation in the U.S.
The coke-deposited catalyst drops by gravity to fill the lower portion of the disengaging vessel where it is stripped with an inert gas such as steam to remove entrained hydrocarbons. The catalyst is then sent to a regenerator vessel for removal of the solid coke which built up on the catalyst during the cracking reaction. In the regenerator, the coke is removed by passing an oxygen-containing gas such as air through the catalyst bed so that the coke burns off as carbon dioxide and carbon monoxide. The regenerated catalyst is then reintroduced into the riser reactor.
When a gas oil is catalytically cracked, the product mix typically contains about 50 volume percent of a product boiling in the gasoline range. This product typically has an octane number in the range of about 80 to 90. With the addition of tetraethyl lead, a high octane product is obtained which is then blended with other gasoline-boiling fractions to produce a gasoline of the desired octane number.
During the past decade and a half, there has been increased concern about the environmental effects of burning gasoline in internal combustion engines. While the major products of the burning are water and carbon dioxide, which are harmless, the products unfortunately include carbon monoxide, sulfur oxides, nitrogen oxides, unburned hydrocarbons, and lead, all of which are air pollutants. Beginning in 1974, most of the automobiles sold in the United States have been equipped with exhaust gas catalytic converters to reduce emissions of carbon monoxide, nitrogen oxides, and unburned hydrocarbons. Since lead poisons the catalyst used in these converters, the addition of lead to gasolines for use in converter-equipped automobiles has been halted.
Since lead addition had been an easy way to increase octane, the oil refining industry is now faced with the problem of developing new processes to produce fractions with intrinsically high octane numbers. The oil refining industry is also faced with the problem of reducing the amount of sulfur in gasoline so as to reduce the emissions of sulfur oxides.